hwclock: fix fgets unchecked return value warning on alpha

hwclock: fix fgets unchecked return value warning on alpha

Build warning:
sys-utils/hwclock-cmos.c: In function 'is_in_cpuinfo':
sys-utils/hwclock-cmos.c:162:4: warning: ignoring return value of 'fgets', declared with attribute warn_unused_result [-Wunused-result]
    fgets(field, 256, cpuinfo);

Full build log:
https://buildd.debian.org/status/fetch.php?pkg=util-linux&arch=alpha&ver=2.26.2-9&stamp=1440078034

Detected by/via:
https://qa.debian.org/bls/packages/u/util-linux.html

This change has never actually been (build-)tested on alpha, but
hopefully the change should fix the warning.

Signed-off-by: Andreas Henriksson <andreas@fatal.se>